The long-term objective is to determine the impact of environmental toxicants on the olfactory system and the role of chemical exposure in the etiology of anosmia and other diseases concerning the nasal cavity, such as nasal polyps, chronic nasal inflammation, microbial infection, and asthma. Our focus has been on the mechanisms of tissue-specific toxicity and molecular approaches to risk assessment. Previous in vitro studies have suggested that olfactory mucosa (OM)-predominant cytochrome P450 (CYP) enzymes may be important in the OM-selective toxicity of many chemicals. In the current funding period, we have succeeded in the preparation and initial characterization of two knockout mouse models, one with germ-line disruption of the OM-specific Cyp2gl gene (Cyp2gl-null mouse), the other with liver-specific deletion of the NADPH-cytochrome P450 reductase (Cpr) gene (liver-Cpr-null mouse). Using these models, we have obtained important preliminary data concerning the role of hepatic metabolism in nasal toxicity of acetaminophen (as a model toxicant), and the role of CYP2A5 and/or CYP2G1 in the toxicity of acetaminophen in the lateral nasal gland (LNG). In the proposed studies, we will test a general hypothesis that target tissue metabolic activation is critical for extrahepatic chemical toxicity;however, the sensitivity of a particular tissue to a given compound will depend on many factors (in addition to the rate of systemic clearance of the parent compound), such as the level and reactivity of circulating hepatic metabolites, and homeostasis of endogenous substrates in the target tissue. The specific aims are: 1) To determine the impact of hepatic metabolism and target tissue metabolic activation on OM toxicity of nasal toxicants;2) To determine the mechanisms of the increased resistance to AP toxicity in the LNG of the Cyp2gl-null mouse;and 3) To generate additional mouse models that can be used for determining the specific roles of CYP2A5 and CYP2G1 in target tissue metabolic activation and chemical toxicity in the OM and LNG. These studies will have broad implications for risk assessment of potential nasal toxicants, and may serve as a model for studying mechanisms of chemical toxicity in other extrahepatic tissues.